This invention relates to digital radio generally, and in particular, to digital radio systems having a digital radio device which performs quadrature demodulation.
A prior art digital radio device has an analog receiving part for receiving phase or frequency modulated radio frequency signals, which receiving part is coupled to frequency down conversion means for obtaining an intermediate frequency signal from the radio frequency signal, the digital radio device further comprising a quadrature demodulator for demodulating the intermediate frequency signal, the quadrature demodulator having a phase shifting network in a quadrature branch. Such a system can be a pagings system, a cellular mobile radio system, a cordless local area network telephony system, a cordless (LAN); or the like. In case of a paging system, the digital radio device usually only comprises a receiver part, and in case of cellular radio, or cordless telephony, or the like, the digital radio device is a transceiver, also comprising a transmitter part.
A digital radio device of this kind is known from the Japanese Abstract No. 1-20706 describing a quadrature detecting circuit for input frequency modulated (FM) signals. The quadrature detector comprises a phase shifting network in a quadrature branch. The phase shifting network comprises a damping resistor parallel to a reactive element to ground, at an output side of the phase shifting network. Such a configuration is disadvantageous in that for demodulation of digital signals asymmetric signal behaviour occurs for respective data to be demodulated, because of an amplitude roll-off at 90xc2x0 phase shift. This problem is the more serious when the filtered signal already contains intersymbol interference, such as in the case of Guassian Frequency Shift Keying (GFSK) signals with a BT-product of 0.5 that are applied in a (DECT) system or a Digital European Cordless Telecommunications NA/WCPE system. Furthermore, when assuming a white noise distribution over the relevant bandwidth, the mean noise level is shifted to a frequency where the phase shift deviates from 90xc2x0. This leads to a direct current (DC) shift in either one of the detected data. When applying AFC (Automatic Frequency Control) on the basis of such detected data, such an AFC operates with a wrong signal. Particularly, in a system with a low signal-to-noise level, such as a paging system, DC-level shift of data might become a severe problem to overcome.
It is an object of the present invention to provide a digital radio system comprising a digital radio device not having the drawbacks of the known system.
To this end a digital radio device according to the present invention is characterised in that the phase shifting network comprises a series arrangement or a parallel arrangement of a first resistor and a reactive element of a first type, which arrangement is coupled to a reactive element of a second type, a junction of the series arrangement and the reactive element of the second type being an output of the phase shifting network providing a quadrature signal, whereby the first resistor provides a damping such that an overall quality factor of the phase shifting network is substantially smaller than a quality factor of the reactive element of the second type. The present invention is based upon the insight that in the configuration according to the present invention a desired overall damping can be achieved while having minimal amplitude roll-off at 90xc2x0 phase shift because for phase shifting the voltage transfer characteristic of the phase shifting network is independent of the first resistor.
In an embodiment of a digital radio device according to the present invention a second resistor is coupled between the first resistor and ground so as to allow for independent adjustment of the overall quality and a gain of the quadrature branch. Because the demodulator is usually fed by a voltage source with a given source resistance only the first resistor would not provide such an independent adjustment. The second resistor provides an additional degree of freedom in adjusting said parameters.
In still further embodiments of a digital radio device according to the present invention, the first reactive element can be a capacitor and the second reactive element can be an inductor, or vice versa. Applying standard network transformations for conversion of a serial network into a parallel network at a given frequency, such as described in network handbooks, a series arrangement of the first resistor and the reactive element of the first type can be converted to a parallel arrangement, or vice versa. In an embodiment of the present invention, the digital radio device can have a receiver branch and a transmitter branch so as to form a transceiver that can be used in cellular radio systems or cordless telephony systems.